 class ListNode {
    int val;
      ListNode next;
      ListNode() {}
      ListNode(int val) { this.val = val; }
      ListNode(int val, ListNode next) { this.val = val; this.next = next; }
}
public class Solution2 {
    //21. 合并两个有序链表
    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
        if (list1 == null) {
            return list2;
        }
        if (list2 == null) {
            return list1;
        }
        if (list1.val <= list2.val) {
            list1.next = mergeTwoLists(list1.next,list2);
            return list1;
        } else {
            list2.next = mergeTwoLists(list1,list2.next);
            return list2;
        }
    }
}
